cross-posted from: https://lemmy.run/post/10868

Beginner’s Guide to grep

grep is a powerful command-line tool used for searching and filtering text in files. It allows you to find specific patterns or strings within files, making it an invaluable tool for developers, sysadmins, and anyone working with text data. In this guide, we will cover the basics of using grep and provide you with some useful examples to get started.

Installation

grep is a standard utility on most Unix-like systems, including Linux and macOS. If you’re using a Windows operating system, you can install it by using the Windows Subsystem for Linux (WSL) or through tools like Git Bash, Cygwin, or MinGW.

Basic Usage

The basic syntax of grep is as follows:

grep [options] pattern [file(s)]
  • options: Optional flags that modify the behavior of grep.
  • pattern: The pattern or regular expression to search for.
  • file(s): Optional file(s) to search within. If not provided, grep will read from standard input.

Examples

Searching in a Single File

To search for a specific pattern in a single file, use the following command:

grep "pattern" file.txt

Replace "pattern" with the text you want to search for and file.txt with the name of the file you want to search in.

Searching in Multiple Files

If you want to search for a pattern across multiple files, use the following command:

grep "pattern" file1.txt file2.txt file3.txt

You can specify as many files as you want, separating them with spaces.

Ignoring Case

By default, grep is case-sensitive. To perform a case-insensitive search, use the -i option:

grep -i "pattern" file.txt

Displaying Line Numbers

To display line numbers along with the matching lines, use the -n option:

grep -n "pattern" file.txt

This can be helpful when you want to know the line numbers where matches occur.

Searching Recursively

To search for a pattern in all files within a directory and its subdirectories, use the -r option (recursive search):

grep -r "pattern" directory/

Replace directory/ with the path to the directory you want to search in.

Using Regular Expressions

grep supports regular expressions for more advanced pattern matching. Here’s an example using a regular expression to search for email addresses:

grep -E "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}\b" file.txt

In this case, the -E option enables extended regular expressions.

Conclusion

grep is a versatile tool that can greatly enhance your text searching and filtering capabilities. With the knowledge you’ve gained in this beginner’s guide, you can start using grep to quickly find and extract the information you need from text files. Experiment with different options and explore more advanced regular expressions to further expand your skills with grep. Happy grepping!

    • rootOP
      4•1 year ago

      I did not.

      Thank you for sharing it. Something you learn everyday, eh 😀.

  • @[email protected]
    6•1 year ago

    This is cool, but I’d love to see some examples outputs so I can get a sense of what it actually does.

  • PenguinCoder
    4•1 year ago

    Good information and barely scratches the surface of grep usage. It can get a lot more complicated but also do a lot more than you think.

    Two of my most used grep invocations are:

    • Diff two files, showing lines: grep -xvFf (file2) (file1)
    • Show lines that do not contain a string: grep -rivE '^#' (file) ( only shows uncommented lines for most Linux configuration files)
  • @[email protected]
    4•1 year ago

    Excellent guide, thanks for the write up!

    One thing I’d like to point out is that you can pipe output from an application into grep and then be able to use all the information above.

    For instance if I want to know the full name of my wireguard interface I can just pass (pipe) the output of ifconfig into grep:

    ifconfig | grep wire

  • @[email protected]
    3•1 year ago

    A few more options that I use pretty much daily:

    • -v for reversing the match (display non-matching instead of matching)
    • -e to specify multiple patterns, matching any one is sufficient
    • -w to match only at word boundaries, easier to type than the equivalent regexp
    • -c for displaying the match count rather than the matches

    And some that that I use occasionally:

    • -NUM, -B NUM, -A NUM also show NUM lines around/before/after match
    • -l to display only the filenames with the matches
    • -F “fixed” pattern meaning literal match only rather than regexp. Great to avoid having to quote regexp special characters when you don’t need regexp matching
    • -P for PCRE style regexps
    • -f to read match patterns from a file
    • -q quiet, only produce exit status, no output. Useful in shell expressions (scripts, one liners).
    • -a force treating the input as text (useful to override the binary detection heuristic. mnemonic: ascii)
  • @TacoDog
    2•1 year ago

    Would you mind explaining this regular expression a little, and how it’s grabbing email addresses? What is “\b”? What does “+” do? And the {2,}?

    • @[email protected]
      2•1 year ago

      + means that of the prepending selection of characters (those in the []-brackets) at least one character has to Match.

      The curly braces are some advanced Form of that and means for {n,m} anything between n and m characters of the prepending selection. In this Case it would be at least 2 as m isn’t given. This will Match any Domain ending With at least 2 characters.

      Notice however that this is a very Basic regex pattern to Match Email addresses.

      • @TacoDog
        1•1 year ago

        I always though you needed to put a star (*) after the brackets to match multiple of the same pattern. The pattern “.*” is multiple of anything right? Been using Linux for 20 years and I still need to look this up every time. There seems to be different regex patterns for bash (where just “*” is multiple of anything) than for sed, awk and grep. I’ve still never seen the \b syntax.

    • Noodlez
      1•1 year ago

      I highly recommend reading man 7 regex or if you’re a more visual and play with learner, use RegExr. It’s even better if you use the two together.

  • TunaCowboy
    2•1 year ago

    Beginner’s Guide to grep

    man grep

  • @[email protected]
    1•1 year ago

    Fantastic summary of one of the most universally used cli tool. One thing to add is that the name grep comes from the ed command g/re/p which stands for “global, regular expression, print” since thats what it does; search everything (global) for a given regular expression (even if the “regular expression” is just a specific string to match) and prints it out. Keeping the name origin in mind helps me to remember what it does.

  • @progenyofthestars
    1•1 year ago

    Thank you, always fun to read these and learn about an option you haven’t used yourself yet.

  • @[email protected]
    1•1 year ago

    If you are serious about working in a terminal, then I highly recommend learning modern replacements for the old tools.

    In this case ripgrep (or rg) https://github.com/BurntSushi/ripgrep is phenomenal. Especially for searching recursively in a large directory tree it is unbelievably quicker than regular grep.

    It won’t be installed on any random machine, so grep is still useful, but if you regularly need to text search in files then there are better tools.

    • rootOP
      3•1 year ago

      Yeap, but most of the time you end up trying to figure out issue on remote system, where you don’t have ripgrep always installed, but if you have that available on the system you are working on. ripgrep is always a better alternative.